"Density and Building Height"
To amplify Atrios’ point a bit, while of course taller buildings will make an area denser, you don’t actually need buildings to be particularly tall in order to have walkable urbanism.
This video from Dan Zack indicates that people are actually pretty bad at guesstimating the residential density of various areas, primarily because of an over-reliance on building height as a cue:
A lot of extremely dense areas, like central Paris, feature basically no super-tall buildings. Or I don’t think that if you walk around Somerville, Massachusetts it will be obvious to you that it’s one of the densest municipalities in America. The issue in both cases is that buildings occupy a very large fraction of the available space—you don’t have many wide streets, parking lots, setback buildings, interior courtyards, etc. For rowhouse neighborhoods, in particular, I’m coming around to the view that setbacks and unused front yards are sort of the silent killer.